Internal functionalization of a single-walled nanotube (SWNT) is an attractive, yet difficult challenge in nanotube materials chemistry. A still unsolved problem is how to immobilize functional moieties on its inner surface through attachment of the functional moieties to the interior surface of the inner pore wall. Doing so, particularly via a covalent bonding, would allow a number of new ways to control the properties of the SWNT and enable use of SWNTs in many applications, including those requiring molecular recognition that exploits the 1-D porosity of SWNTs (e.g., catalysis, adsorption, membranes, sensors).
The most well-known SWNT is the single-walled carbon nanotube (CNT), first prepared in 1991. The formation of covalent bonds at the CNT surface requires a transformation of carbon hybridization from sp2 to sp3. While this can be accomplished on the outer surface, the process requires very harsh reaction conditions. To date there has been no demonstration of interior functionalization of CNTs by the formation of covalent bonds on its interior surface or within its pore. Because the interior surface of a CNT is concave, it provides an extremely high thermodynamic obstacle for transformation. Thus, the interior surfaces of CNTs have been considered essentially unreactive. Some recent work indicates that the interior surface of CNTs may become reactive but, again, the reactivity was only made possible under very specific and very extreme conditions, a finding that further corroborates the essentially unreactive nature of the inner walls of CNTs.
Synthetic metal oxide nanotubes can potentially overcome some of the limitations observed with CNTs. An example of these nanotubes is a synthetic aluminosilicate SWNT. While some minor modifications to synthetic aluminosilicate nanotubes have been performed, the modifications were generally achieved only after synthesis via grafting, requiring a multi-step process post-synthesis and did not provide uniform distribution of the functional group along the length of the nanotube. In fact, the functionalization observed post-synthesis was only possible at the nanotube mouth or opening of the pore and the incorporation of the functional group was in line with the nanotube wall. In aluminosilicate nanotubes having inner pore diameters of 2 nm or greater, some limited functionalization was also performed; however, functionalization imparted only hydrophobicity to the inner surface of the inner pore wall and modification was highly limited to a hydrophobic methyl group, which could not be replicated with other charged functional moieties. Thus, efforts to provide functionalized SWNTs and to improve the methods for functionalization of SWNTs remain desirable.